Method of clutch control to start an engine with a hybrid transmission

ABSTRACT

A method of starting an internal combustion engine includes selecting a first or a second clutch, and pressurizing the selected clutch with an auxiliary pump. The pressurized clutch is synchronized using the first and second electric machines, and then engaging. The internal combustion engine is started with the first and second electric machines. The first and second clutches may be engine reactive clutches. Selecting the first or second clutch includes determining whether an input-split or compound-split mode is requested. If the input-split mode is requested the first clutch is selected, and if the compound-split mode is requested the second clutch is selected. The first and second electric machines may be high voltage machines, configured to operate in conjunction with a high-voltage battery pack. The first and second electric machines may be configured as propulsion motors for the vehicle.

TECHNICAL FIELD

The present invention relates generally to hybrid powertrains formotorized vehicles, and hydraulic control thereof.

BACKGROUND OF THE INVENTION

Motorized vehicles include a powertrain operable to propel the vehicleand power the onboard vehicle electronics. The powertrain, ordrivetrain, generally includes an engine that powers the final drivesystem through a multi-speed power transmission. Many vehicles arepowered by a reciprocating-piston type internal combustion engine (ICE).

Hybrid vehicles utilize alternative power sources to propel the vehicle,minimizing reliance on the engine for power. A hybrid electric vehicle(HEV), for example, incorporates both electric energy and chemicalenergy, and converts the same into mechanical power to propel thevehicle and power the vehicle systems. The HEV generally employs one ormore electric machines that operate individually or in concert with aninternal combustion engine to propel the vehicle. Since hybrid vehiclescan derive their power from sources other than the engine, engines inhybrid vehicles may be turned off while the vehicle is stopped or isbeing propelled by the alternative power source(s).

Parallel hybrid architectures are generally characterized by an internalcombustion engine and one or more electric motor/generator assemblies,all of which have a direct mechanical coupling to the transmission.Parallel hybrid designs utilize combined electric motor/generators,which provide traction and may replace both the conventional startermotor and alternator. The motor/generators are electrically connected toan energy storage device (ESD). The energy storage device may be achemical battery. A control unit is employed for regulating theelectrical power interchange between the energy storage device andmotor/generators, as well as the electrical power interchange betweenthe first and second motor/generators.

Electrically-variable transmissions (EVT) provide for continuouslyvariable speed ratios by combining features from both series andparallel hybrid powertrain architectures, and also elements oftraditional, non-hybrid transmissions. EVTs may be designed to operatein both fixed-gear (FG) modes and EVT modes. When operating in afixed-gear mode, the rotational speed of the transmission output memberis a fixed ratio of the rotational speed of the input member from theengine, depending upon the selected arrangement of the differentialgearing subsets. EVTs may also be configured for engine operation thatis mechanically independent from the final drive.

The EVT can utilize the differential gearing to send a fraction of itstransmitted power through the electric motor/generator(s) and theremainder of its power through another, parallel path that ismechanical. One form of differential gearing used is the epicyclicplanetary gear arrangement. However, it is possible to design a powersplit transmission without planetary gears, for example, as by usingbevel gears or other differential gearing.

Hydraulically-actuated torque-transmitting mechanisms, such as clutchesand brakes, are selectively engageable to selectively activate the gearelements for establishing different forward and reverse speed ratios andmodes between the transmission input and output shafts. The term“clutch” is used hereinafter to refer generally to torque transmittingmechanisms, including, without limitation, devices commonly referred toas clutches and brakes. Shifting from one speed ratio or mode to anothermay be in response to vehicle conditions and operator (driver) demands.The “speed ratio” is generally defined as the transmission input speeddivided by the transmission output speed. Thus, a low gear range has ahigh speed ratio, and a high gear range has a relatively lower speedratio. Because EVTs are not limited to single-speed gear ratios, thedifferent operating states may be referred to as ranges or modes.

SUMMARY OF THE DISCLOSURE

A method of starting an internal combustion engine is provided. Theinternal combustion engine is within a vehicle having a first electricmachine and a second electric machine incorporated into a multi-modehybrid transmission. The method includes selecting a first clutch or asecond clutch, and pressurizing the selected clutch with an auxiliarypump. The method further includes synchronizing the pressurized clutchusing at least one of the first and second electric machines, andengaging the pressurized clutch. The internal combustion engine is thenstarted with at least one of the first and second electric machines.

The first and second clutches may be engine reactive clutches. Selectingeither the first or second clutch includes determining whether aninput-split mode or a compound-split mode is requested. If theinput-split mode is requested the first clutch is selected, and if thecompound-split mode is requested the second clutch is selected.Synchronizing the pressurized clutch may include using both of the firstand second electric machines.

The internal combustion engine may be started by using both the firstand the second electric machines. The first and second electric machinesmay be high voltage machines, configured to operate in conjunction witha high-voltage battery pack. The first and second electric machines mayalso be configured as propulsion motors for the vehicle.

The above features and advantages, and other features and advantages ofthe present invention will be readily apparent from the followingdetailed description of the preferred embodiments and other modes forcarrying out the present invention when taken in connection with theaccompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic lever diagram illustration of an exemplary vehiclepowertrain with a multi-mode, electrically-variable hybrid transmissionin accordance with the present invention;

FIG. 2 is a truth table listing the engaged torque-transmittingmechanisms for each of the operating modes of the transmissionillustrated in FIG. 1;

FIG. 3 is a graphical representation of various regions of operationwith respect to input and output speeds of the transmission illustratedin FIG. 1; and

FIG. 4 is a flow chart or block diagram illustrating an exemplary clutchcontrol method or algorithm in accordance with the claimed invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The claimed invention is described herein in the context of ahybrid-type vehicular powertrain having a multi-mode, multi-speed,electrically-variable, hybrid transmission, which is intended solely tooffer a representative application by which the present invention may beincorporated and practiced. The claimed invention is not limited to theparticular powertrain arrangement shown in the drawings. Furthermore,the hybrid powertrain illustrated herein has been greatly simplified, itbeing understood that further information regarding the standardoperation of a hybrid powertrain, or a hybrid-type vehicle will berecognized by those having ordinary skill in the art.

Referring to the drawings, wherein like reference numbers refer to likecomponents throughout the several views, there is shown in FIG. 1 alever diagram depiction of an exemplary vehicle powertrain system,designated generally as 10. The powertrain 10 includes a restartableengine 12 that is selectively drivingly connected to, or in power flowcommunication with, a final drive system 16 via a multi-mode,electrically-variable hybrid-type power transmission 14.

A lever diagram is a schematic representation of the components of amechanical device such as an automatic transmission. Each individuallever represents a planetary gearset, wherein the three basic mechanicalcomponents of the planetary gear are each represented by a node.Therefore, a single lever contains three nodes: one for the sun gearmember, one for the planet gear carrier member, and one for the ringgear member. The relative length between the nodes of each lever may beused to represent the ring-to-sun ratio of each respective gearset.These lever ratios, in turn, are used to vary the gear ratios of thetransmission in order to achieve appropriate ratios and ratioprogression. Mechanical couplings or interconnections between the nodesof the various planetary gear sets and other components of thetransmission (such as motor/generators) are illustrated by thin,horizontal lines. Torque transmitting devices such as clutches andbrakes are presented as interleaved fingers. If the device is a brake,one set of the fingers is grounded.

The transmission 14 is designed to receive at least a portion of itsdriving power from the engine 12, through an input member 18, forexample. The transmission input member 18, which is in the nature of ashaft, may be the engine output shaft (also referred to as a“crankshaft”). Alternatively, a transient torque damper (not shown) maybe implemented between the engine 12 and the input member 18 of thetransmission 14. The engine 12 transfers power to the transmission 14,which distributes torque through a transmission output member or shaft20 to drive the final drive system 16, and thereby propel the vehicle(not shown).

In the embodiment depicted in FIG. 1, the engine 12 may be any ofnumerous forms of petroleum-fueled prime movers, such as thereciprocating-piston type internal combustion engines, which includesspark-ignited gasoline engines and compression-ignited diesel engines.The engine 12 is readily adaptable to provide its available power to thetransmission 14 at a range of operating speeds, for example, from idle,at or near 600 revolutions per minute (RPM), to over 6,000 RPM.Irrespective of the means by which the engine 12 is connected to thetransmission 14, the input member 18 is connected to a differential gearset encased within the transmission 14, as explained in more detailherein.

Referring still to FIG. 1, the hybrid transmission 14 utilizes one ormore differential gear arrangements, preferably in the nature of threeinterconnected epicyclic planetary gear sets, designated generally at24, 26 and 28, respectively. Each gear set includes three gear members:a first, second and third member. In referring to the first, second andthird gear sets in this description and in the claims, these sets may becounted “first” to “third” in any order in the drawings (e.g., left toright, right to left, etc.). Likewise, in referring to the first, secondand third members of each gear set in this description and in theclaims, these members may be counted or identified as “first” to “third”in any order in the drawings (e.g., top to bottom, bottom to top, etc.)for each gear set.

The first planetary gear set 24 has three gear members: a first, secondand third member 30, 32 and 34; respectively. In a preferred embodiment,the first member 30 includes of an outer gear member (which may bereferred to as a “ring gear”) that circumscribes the third member 34,which may include of an inner gear member (which may be referred to as a“sun gear”). In this instance, the second member 32 acts as a planetcarrier member. That is, a plurality of planetary gear members (whichmay be referred to as “pinion gears”) are rotatably mounted on thesecond member, planet carrier 32. Each planetary gear member ismeshingly engaged with both the first member, ring gear 30 and the thirdmember, sun gear 34.

The second planetary gear set 26 also has three gear members: a first,second and third member 40, 42 and 44, respectively. In the preferredembodiment discussed above with respect to the first planetary gear set24, the first member 40 of the second planetary gear set 26 is an outer“ring” gear member that circumscribes the third member 44, which is aninner “sun” gear member. The ring gear member 40 is coaxially alignedand rotatable with respect to the sun gear member 44. A plurality ofplanetary gear members are rotatably mounted on the second member 42,which acts as a planet carrier member, such that each planetary gearmeshingly engages both the ring gear member 40 and the sun gear member44.

The third planetary gear set 28, similar to the first and second gearsets 24, 26, also has first, second and third members 50, 52 and 54,respectively. In this arrangement, however, the second member 52, shownon the middle node of the lever representing the third planetary gearset 28, is the outer “ring” gear. The ring gear (second member 52) iscoaxially aligned and rotatable with respect to the sun gear, thirdmember 54. The first member 50 is the planet carrier in this particulargear set, and is shown on the top node. As such, a plurality ofplanetary or pinion gear members are rotatably mounted on the planetcarrier, first member 50. Each of the pinion gear members is aligned tomeshingly engage either the ring gear (second member 52) and an adjacentpinion gear member or the sun gear (third member 54) and an adjacentpinion gear member.

In one embodiment, the first and second planetary gear sets 24, 26 eachcomprise simple planetary gear sets, whereas the third planetary gearset 28 comprises a compound planetary gear set. However, each of theplanet carrier members described above can be either a single-pinion(simple) carrier assembly or a double-pinion (compound) carrierassembly. Embodiments with long pinions are also possible.

The first, second and third planetary gear sets 24, 26, 28 arecompounded in that the second member 32 of the first planetary gear set24 is conjoined with (i.e., continuously connected to) the second member42 of the second planetary gear set 26 and the third member 54 of thethird planetary gear set 28, as by a central shaft 36. As such, thesethree gear members 32, 42, 54 are rigidly attached for common rotation.

The engine 12 is continuously connected to the first planetary gear set24, namely first member 30, for example, through an integral hub plate38, for common rotation therewith. The third member 34 of the firstplanetary gear set 24 is continuously connected, for example, by a firstsleeve shaft 46, to a first motor/generator assembly 56, which is alsoreferred to herein as “motor A”. The third member 44 of the secondplanetary gear set 26 is continuously connected, for example, by asecond sleeve shaft 48, to a second motor/generator assembly 58, alsoreferred to herein as “motor B”. The second member 52 (the ring gear) ofthe third planetary gear set 28 is continuously connected totransmission output member 20, for example, through an integral hubplate. The first and second sleeve shafts 46, 48 may circumscribe thecentral shaft 36.

A first torque transfer device 70—which is herein interchangeablyreferred to as clutch “C1”—selectively connects the first gear member 50with a stationary member, represented in FIG. 1 by transmission housing60. The second sleeve shaft 48, and thus gear member 44 andmotor/generator 58, is selectively connectable to the first member 50 ofthe third planetary gear set 28 through the selective engagement of asecond torque transfer device 72—which is herein interchangeablyreferred to as clutch “C2”. A third torque transfer device 74—which isherein interchangeably referred to as clutch “C3”—selectively connectsthe first gear member 40 of the second planetary gear set 26 to thetransmission housing 60. The first sleeve shaft 46, and thus third gearmember 34 and first motor/generator 56, is also selectively connectableto the first member 40 of the second planetary gear set 26, through theselective engagement of a fourth torque transfer device 76—which isherein interchangeably referred to as clutch “C4”.

A fifth torque transfer device 78—which is herein interchangeablyreferred to as clutch “C5”—selectively connects the input member 18 ofengine 12 and the first gear member 30 of the first planetary gear set24 to the transmission housing 60. Clutch C5 is an input brake clutch,which selectively locks the input member 18 when engine 12 is off.Locking input member 18 provides more reaction for regenerative brakingenergy. As shown below, in reference to FIG. 2, C5 is not involved inthe mode/gear/neutral shifting maneuvers of transmission 14.

The first and second torque transfer devices 70, 72 (C1 and C2) may bereferred to as “output clutches.” The third and fourth torque transferdevices 74, 76 (C3 and C4) may be referred to as “holding clutches”.

In the exemplary embodiment depicted in FIG. 1, the various torquetransfer devices 70, 72, 74, 76, 78 (C1-C5) are all friction clutches.However, other conventional clutch configurations may be employed, suchas dog clutches, rocker clutches, and others recognizable to thosehaving ordinary skill in the art. The clutches C1-C5 may behydraulically actuated, receiving pressurized hydraulic fluid from apump (not shown). Hydraulic actuation of clutches C1-C5 is accomplished,for example, by using a conventional hydraulic fluid control circuit, aswill be recognized by one having ordinary skill in the art.

In the exemplary embodiment described herein, wherein the hybridpowertrain 10 is used as a land vehicle, the transmission output shaft20 is operatively connected to the final drive system (or “driveline”).The driveline may include a front or rear differential, or other torquetransfer device, which provides torque output to one or more wheelsthrough respective vehicular axles or half-shafts (not shown). Thewheels may be either front or rear wheels of the vehicle on which theyare employed, or they may be a drive gear of a track vehicle. Thosehaving ordinary skill in the art will recognize that the final drivesystem may include any known configuration, including front wheel drive(FWD), rear wheel drive (RWD), four-wheel drive (4WD), or all-wheeldrive (AWD), without altering the scope of the claimed invention.

All of the planetary gear sets 24, 26, 28, as well as the first andsecond motor/generators 56, 58 (motor A and motor B,) are preferablycoaxially oriented about the intermediate central shaft 36 or anotheraxis. Motor A or motor B may take on an annular configuration,permitting one or both to generally circumscribe the three planetarygear sets 24, 26, 28. Such a configuration may reduce the overallenvelope, i.e., the diametrical and longitudinal dimensions, of thehybrid transmission 14 are minimized.

The hybrid transmission 14 receives input motive torque from a pluralityof torque-generative devices. “Torque-generative devices” include theengine 12 and the motors/generators 56, 58 as a result of energyconversion from fuel stored in a fuel tank or electrical potentialstored in an electrical energy storage device (neither of which isshown).

The engine 12, motor A (56,) and motor B (58) may operate individuallyor in concert—in conjunction with the planetary gear sets andselectively-engageable torque-transmitting mechanisms—to rotate thetransmission output shaft 20. Moreover, motor A and motor B arepreferably configured to selectively operate as both a motor and agenerator. For example, motor A and motor B are capable of convertingelectrical energy to mechanical energy (e.g., during vehiclepropulsion), and further capable of converting mechanical energy toelectrical energy (e.g., during regenerative braking or during periodsof excess power supply from engine 12).

With continuing reference to FIG. 1, an electronic control apparatus (or“controller”) having a distributed controller architecture is shownschematically in an exemplary embodiment as a microprocessor-basedelectronic control unit (ECU) 80. The ECU 80 includes a storage mediumwith a suitable amount of programmable memory, collectively representedat 82, that is programmed to include, without limitation, an algorithmor method 100 of regulating operation of a multi-mode hybridtransmission, as will be discussed in further detail below with respectto FIG. 4.

The control apparatus is operable, as described hereinafter, to providecoordinated system control of the powertrain 10 schematically depictedand described herein. The constituent elements of the control apparatusmay be a subset of an overall vehicle control system. The control systemis operable to synthesize pertinent information and inputs, and executecontrol methods and algorithms to control various actuators to achievecontrol targets. The control system monitors target and parametersincluding, without limitation: fuel economy, emissions, performance,driveability, and protection of drivetrain hardware—such as, but notlimited to, the engine 12, transmission 14, motor A, motor B, and finaldrive 16.

The distributed controller architecture (ECU 80) may include aTransmission Control Module (TCM), an Engine Control Module (ECM), aTransmission Power Inverter Module (TPIM), and a Battery Pack ControlModule (BPCM). A Hybrid Control Module (HCP) may be integrated to offeroverall control and coordination of the aforementioned controllers.

A User Interface (UI) is operatively connected to a plurality of devices(not shown) through which a vehicle operator typically controls ordirects operation of the powertrain. Exemplary vehicle operator inputsto the user interface include an accelerator pedal, a brake pedal,transmission gear selector, vehicle speed cruise control, and otherinputs recognizable to those having ordinary skill in the art.

Each of the aforementioned controllers communicates with othercontrollers, sensors, actuators, etc., via a control area network (CAN)bus or communication architecture. The CAN bus allows for structuredcommunication of control parameters and commands between the variouscontrollers. The communication protocol utilized isapplication-specific. For example, and without limitation, one useablecommunication protocol is the Society of Automotive Engineers standardJ1939. The CAN bus and appropriate protocols provide for robustmessaging and multi-controller interfacing between the aforementionedcontrollers, and other controllers providing functionality such asantilock brakes, traction control, and vehicle stability.

The engine control module is operatively connected to, and incommunication with, the engine 12. The engine control module isconfigured to acquire data from a variety of sensors and control avariety of actuators of the engine 12 over a plurality of discretelines. The engine control module receives an engine torque command fromthe hybrid control module, generates a desired axle torque, and anindication of actual engine torque, which is communicated to the hybridcontrol module. Various other parameters that may be sensed by theengine control module include engine coolant temperature, engine inputspeed to the transmission, manifold pressure, and ambient airtemperature and pressure. Various actuators that may be controlled bythe engine control module include, without limitation, fuel injectors,ignition modules, and throttle control modules.

The transmission control module is operatively connected to thetransmission 14, and functions to acquire data from a variety of sensorsand provide command signals to the transmission 14. Inputs from thetransmission control module to the hybrid control module may includeestimated clutch torques for each of the clutches C1-C5, and rotationalspeed of the transmission output shaft 20. Additional actuators andsensors may be used to provide additional information from thetransmission control module to the hybrid control module for controlpurposes.

Each of the aforementioned controllers may be a general-purpose digitalcomputer, generally including a microprocessor or central processingunit, read only memory (ROM), random access memory (RAM), electricallyprogrammable read only memory (EPROM), high speed clock, analog todigital (A/D) and digital to analog (D/A) circuitry, and input/outputcircuitry and devices (I/O) and appropriate signal conditioning andbuffer circuitry. Each controller has a set of control algorithms,including resident program instructions and calibrations stored in ROMand executed to provide the respective functions of each computer.Information transfer between the various computers may be accomplishedusing the aforementioned CAN.

In response to operator input, as captured by the user interface, thesupervisory hybrid control module controller and one or more of theother controllers described above with respect to FIG. 1 determinerequired transmission output torque. Selectively operated components ofthe hybrid transmission 14 are appropriately controlled and manipulatedto respond to the operator demand. For example, in the embodiment shownin FIG. 1, when the operator has selected a forward drive range andmanipulates either the accelerator pedal or the brake pedal, the hybridcontrol module determines an output torque for the transmission, whichaffects how and when the vehicle accelerates or decelerates. Finalvehicle acceleration is affected by other variables, including suchfactors as road load, road grade, and vehicle mass. The hybrid controlmodule monitors the parametric states of the torque-generative devices,and determines the output of the transmission required to arrive at thedesired torque output. Under the direction of the hybrid control module,the transmission 14 operates over a range of output speeds from slow tofast in order to meet the operator demand.

The ECU 80 also receives frequency signals from sensors for processinginto input member 18 speed, N_(i), and output member 20 speed, N_(o),for use in the control of transmission 14. The system controller mayalso receive and process pressure signals from pressure switches (notshown) for monitoring clutch application chamber pressures.Alternatively, pressure transducers for wide range pressure monitoringmay be employed. Pulse-width modulation (PWM) and/or binary controlsignals are transmitted by the controller 80 to transmission 14 forcontrolling fill and drain of clutches C1-C5 for application and releasethereof.

Additionally, the controller 80 may receive transmission fluid sumptemperature data, such as from thermistor inputs (not shown), to derivea sump temperature. Controller 80 may provide PWM signals derived frominput speed, N_(i), and sump temperature for control of line pressurevia one or more regulators.

Fill and drain of clutches C1-C5 may be effectuated, for example, bysolenoid controlled spool valves responsive to PWM and binary controlsignals. Trim valves may be employed using variable bleed solenoids toprovide precise placement of the spool within the valve body andcorrespondingly precise control of clutch pressure during apply.Similarly, one or more line pressure regulators (not shown) may beutilized for establishing regulated line pressure in accordance with thecontrol signal. Clutch slip speeds across clutches may be derived from,for example: transmission input speed, output speed, motor A speed,and/or motor B speed.

The multi-mode, electrically-variable, hybrid transmission 14 isconfigured for several transmission operating modes. The truth tableprovided in FIG. 2 presents an exemplary engagement schedule of thetorque-transmitting mechanisms C1-C4 to achieve the array of operatingstates or modes. The various transmission operating modes described inthe table indicate which of the specific clutches C1-C4 are engaged(actuated), and which are released (deactivated) for each of theoperating modes.

In general, ratio changes in transmission 14 may be performed such thattorque disturbances are minimized, and the shifts are smooth andunobjectionable to the vehicle occupants. Additionally, release andapplication of clutches C1-C4 should be performed in a manner whichconsumes the least amount of energy, and does not negatively impactdurability of the clutches. One major factor affecting theseconsiderations is the torque at the clutch being controlled, which mayvary significantly in accordance with such performance demands asacceleration and vehicle loading. Improved shifts may be accomplished bya zero, or close to zero, reactive torque condition at the clutches atthe time of application or release, which condition followssubstantially zero slip across the clutch. Clutches having zero slipacross the clutch may be referred to as operating synchronously.

Electrically-variable operating modes may be separated into four generalclasses: input-split modes, output-split modes, compound-split modes,and series modes. In an input-split mode, one motor/generator (such aseither motor A or motor B) is geared such that its speed varies indirect proportion to the transmission output, and anothermotor/generator (such as the other of motor A or motor B) is geared suchthat its speed is a linear combination of the input and output memberspeeds. In an output-split mode, one motor/generator is geared such thatits speed varies in direct proportion to the transmission input member,and the other motor/generator is geared such that its speed is a linearcombination of the input member and the output member speeds. Acompound-split mode, however, has both motor/generators geared such thattheir speeds are linear combinations of the input and output memberspeeds, but neither is in direct proportion to either the speed of theinput member or the speed of the output member.

Finally, when operating in a series mode, one motor/generator is gearedsuch that its speed varies in direct proportion to the speed of thetransmission input member, and another motor/generator is geared suchthat its speed varies in direct proportion to the speed of thetransmission output member. When operating in series mode, there is nodirect mechanical power transmission path between the input and outputmembers and therefore all power must be transmitted electrically.

In each of the four general types of electrically-variable operatingmodes indicated above, the speeds of the motors are linear combinationsof the input and output speeds. Thus, these modes have two speed degreesof freedom (which may be abbreviated for simplicity as “DOF”).Mathematically, the torque (T) and speed (N) equations of this class ofmodes take the form:

$\begin{bmatrix}T_{a} \\T_{b}\end{bmatrix} = {{{\begin{bmatrix}a_{1,1} & a_{1,2} \\a_{2,1} & a_{2,2}\end{bmatrix}\begin{bmatrix}T_{i} \\T_{o}\end{bmatrix}}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}N_{a} \\N_{b}\end{bmatrix}}} = {\begin{bmatrix}b_{1,1} & b_{1,2} \\b_{2,1} & b_{2,2}\end{bmatrix}\begin{bmatrix}N_{i} \\N_{o}\end{bmatrix}}}$

where a and b are coefficients determined by the transmission gearing.The type of EVT mode can be determined from the structure of the matrixof b coefficients. That is, if b_(2,1)=b_(1,2)=0 or b_(1,1)=b_(2,2)=0,the mode is a series mode. If b_(1,1)=0 or b_(1,2)=0, the mode is aninput split mode. If b_(2,1)=0 or b_(2,2)=0, the mode is an output splitmode. If each of b_(1,1), b_(1,2), b_(2,1), and b_(2,2) are nonzero, forexample, the mode is a compound split mode.

An electrically-variable transmission may also contain one or morefixed-gear (FG) modes. In general, FG modes result from closing (i.e.,actuating) one additional clutch than the number required to select anelectrically-variable mode. In FG modes, the speed of the input and eachmotor are proportional to the speed of the output. Thus, these modeshave only one speed degree of freedom. Mathematically, the torque andspeed equations of this class of modes take the form:

$\left\lbrack T_{b} \right\rbrack = {{{\begin{bmatrix}a_{1,1} & a_{1,2} & a_{1,3}\end{bmatrix}\begin{bmatrix}T_{a} \\T_{i} \\T_{o}\end{bmatrix}}\mspace{14mu} {{and}\mspace{14mu}\begin{bmatrix}N_{a} \\N_{b} \\N_{i}\end{bmatrix}}} = {\begin{bmatrix}b_{1,1} & b_{1,2} & b_{1,3}\end{bmatrix}\left\lbrack N_{o} \right\rbrack}}$

where a and b are again coefficients determined by the transmissiongearing. If b_(1,1) is nonzero, motor A can contribute to output torqueduring operation in the fixed-gear mode. If b_(1,2) is nonzero, motor Bcan contribute to output torque during operation in the fixed-gear mode.If b_(1,3) is nonzero, the engine can contribute to output torque duringoperation in the fixed-gear mode. If b_(1,3) is zero, the mode is anelectric-only fixed-gear mode.

An electrically-variable transmission may also be configured for one ormore modes with three speed degrees of freedom. These modes may or maynot include reaction torque sources such that the transmission iscapable of producing output torque proportional to engine torque ormotor torque. If a mode with three speed degrees of freedom is capableof producing output torque, the torques of the engine and any motorconnected as a reaction to the engine torque will generally beproportional to the output torque. If a motor is not connected as areaction to the engine torque, its torque can be commanded to controlits speed independently of the transmission input and output speed.

In a mode with three speed degrees of freedom, it is generally notpossible to easily control battery power independently of output torque.This type of mode produces an output torque which is proportional toeach of the reacting torque sources in the system. The fraction of totaloutput power provided by each of the three torque sources may beadjusted by varying the speeds of the motors and input. These modes arehereafter referred to as electric torque converter (ETC) modes inrecognition of the fact that power flows to or from the energy storagedevice as a function of the output torque and the speed of the engine,output, and one of the motors. Mathematically, the torque and speedequations of this class of modes take the form:

$\begin{bmatrix}T_{a} \\T_{b} \\T_{i}\end{bmatrix} = {{{\begin{bmatrix}a_{1,1} & a_{1,2} & a_{1,3}\end{bmatrix}\left\lbrack T_{o} \right\rbrack}\mspace{14mu} {{and}\mspace{14mu}\left\lbrack N_{b} \right\rbrack}} = {\begin{bmatrix}b_{1,1} & b_{1,2} & b_{1,3}\end{bmatrix}\begin{bmatrix}N_{a} \\N_{i} \\N_{o}\end{bmatrix}}}$

where a and b are coefficients determined by the transmission gearing.If a_(1,1) is nonzero, motor A serves as a reaction member and itstorque is proportional to output torque when operating in the ETC mode.If a_(1,1) is zero, motor A is disconnected and its torque is notdetermined by the output torque. If a_(1,2) is nonzero, motor B servesas a reaction member and its torque is proportional to output torquewhen operating in the ETC mode. If a_(1,2) is zero, motor B isdisconnected and its torque is not determined by the output torque. Ifa_(1,3) is nonzero, the engine can contribute to output torque duringoperation in the fixed-gear mode. If a_(1,3) is zero, the input isdisconnected and its torque is not determined by the output torque. Ifall of a_(1,1), a_(1,2), and a_(1,3) are zero, the mode is a neutralmode that is not capable of producing output torque.

There are four neutral modes presented in FIG. 2. In Neutral 1, allclutches are released. Neutral 1 may be utilized when the entire vehicleis stopped and in an off-state, and thus there is no power distribution,electrical, mechanical, or otherwise, being actively distributedthroughout the powertrain 10. In Neutral 1, a 12-voltstarting-lighting-and-ignition (SLI) battery may be used for enginestart.

In Neutral 2, only clutch C3 is engaged, and motor A and motor B mayreact engine 12 for start or to charge the energy storage device.Similar to Neutral 2, when transmission 14 is in Neutral 3, motor A andmotor B may react engine 12 for start or to charge the energy storagedevice, and clutch C4 as the only engaged torque-transmitting device. InNeutral 4, the third and fourth clutches C3, C4 are both in an activatedstate. In this instance, motor A is locked or “grounded”, and motor B isgeared with the engine 12 for engine start.

The first and second planetary gear sets 24, 26 cooperate with the firstand second motor/generators 56, 58, along with the selective engagementof the first and second clutches C1, C2, to constitute an electrictorque converter (ETC). For example, when the transmission 14 isoperating in an ETC mode, the electric output of motor A and/or motor B,depending upon the active control schedule, can be adapted to controlthe transfer of torque from the engine 12 through the transmissiondifferential gearing to the output member 20. When the vehicle isstarted, ETC1 Mode is established by engaging the first clutch C1. InETC1 Mode, motor A reacts engine 12 with the first and third planetarygear sets 24, 28, and motor B freewheels. In ETC1 Mode, the stationaryvehicle can be smoothly started with the engine 12 held at a suitablespeed by gradually increasing the amount of electric power generated bymotor A—i.e., the reaction force of motor A.

There are two other alternative ETC modes available utilizing thetransmission configuration presented herein. ETC2 Mode, also known as“compound ETC”, can be initiated by engaging clutch C2, and disengagingthe remaining clutches. In ETC2 Mode, motor A reacts engine 12 with thefirst and third planetary gear sets 24, 28, while motor B reacts engine12 and motor A to the output member 20. The distribution of enginetorque is manipulated through the cooperative management of the amountof electric power output generated by motor A and motor B.

The third ETC mode, ETC12 Mode, can be initiated by engaging both clutchC1 and clutch C2. Similar to ETC1 Mode, motor A reacts the engine 12with the first and third planetary gear sets 24, 28. However, in thisinstance, motor B is grounded to the transmission housing 60. In ETC12Mode, the vehicle can be smoothly accelerated with the engine 12 held ata suitable speed by gradually increasing the reaction force generated bymotor A; which may be proportional to the electric power generated bymotor A.

When the engine 12 is in an off-state, the transmission 14 can utilizethe ETC mode clutch control schedule to vary the amount of electricenergy generated by motor A so as to gradually increase the drive torqueof motor A and/or motor B. For example, if the transmission 14 isshifted into ETC1 Mode when the engine 12 is in an off-state, the engine12 will create a reaction force, by way of input member 18. The motiveoutput of the motor A can then be controlled, and a continuous anduninterrupted transmission output torque maintained, without having toturn the engine 12 on.

The exemplary powertrain 10 described herein has three fixed-gear (FG),or “direct,” modes of operation. In all fixed-gear modes of thisembodiment of transmission 14, the vehicle is driven in the forwarddirection by operation of the engine 12. The selective engagement ofclutches C1, C3 and C4 shifts the transmission 14 into FG1 Mode. In FG1,motor A is grounded, and the engine drives the first planetary gear set24 to the third planetary gear set 28 and, thus, the output member 20.FG2 Mode is achieved by the selective engagement of clutches C1, C2 andC4. In FG2, motor B is grounded, and the engine drives the first andsecond planetary gear sets 24, 26 to the third planetary gear set 28and, thus, the output member 20. Likewise, FG3 Mode is achieved by theselective engagement of clutches C2, C3 and C4. In FG3, motor A islocked, and the engine drives the first planetary gear set 24 to thesecond and third planetary gear sets 26, 28 and the output member 20.When operating in a fixed-gear mode of operation, the output memberspeed N_(o) is directly proportional to input member speed N_(i) and theselected gear ratio. N_(i)=N_(o)×GR.

With continued reference to FIG. 2, the transmission 14 may also operatein four electrically-variable transmission (EVT) modes. In EVT1 andEVT4, the transmission 14 is operating in an input-split mode ofoperation, wherein the output speed N_(o) of the transmission 14 isproportional to the speed of one motor/generator 56, 58 (motor A ormotor B). Specifically, EVT1 Mode is achieved through the selectiveengagement of the first and third clutches C1 and C3. When in EVT1,motor A functions to react the engine 12 with the first planetary gearset 24, to the third planetary gear set 28, and the output member 20;while motor B drives the second and third planetary gear sets 26, 28.Motor A propels the vehicle in EVT1. Alternatively, the transmission 14may be selectively shifted into EVT4 Mode by actuating clutch C2 andclutch C3. In EVT4, motor A functions to react the engine 12 with thefirst planetary gear set 24, to the second and third planetary gear sets26, 28, and the output member 20, while motor B drives the second andthird planetary gear sets 26, 28. Motor B propels the vehicle in EVT4.

In EVT2 and EVT3, the transmission 14 is operating in a compound-splitmode, wherein the output speed N_(o) of the transmission 14 is notproportional to the speed of a single motor/generator, but is rather analgebraic linear combination of the speeds of both motor/generators.More particularly, EVT2 is achieved through the selective engagement ofthe first and fourth clutches C1, C4. In this mode, motor A and motor Boperate to react the engine 12 with the first and second planetary gearssets. Alternatively, the transmission 14 may be selectively shifted intoEVT3 Mode by actuating clutch C2 and clutch C4. When operating in EVT3Mode, the two motor/generator assemblies 56, 58 react the engine 12 withall three planetary gear sets 24, 26, 28.

With reference to FIG. 3, a plot of transmission output speed, N_(o),along the horizontal axis versus input speed, N_(i), across the verticalaxis is illustrated. FIG. 3 is only a graphical representation ofexemplary regions of operation for each operating mode with respect toinput and output speeds of this embodiment of transmission 14.

Synchronous operation in FG1—the input speed and output speedrelationships where clutches C1, C3 and C4 are operating withsubstantially zero slip speed thereacross—is represented by line 91. Assuch, line 91 represents an input and output speed relationship at whichsubstantially synchronous shifting between EVT modes can occur. FG1 isalso a range at which direct mechanical coupling from input to outputcan be effected by simultaneous application of clutches C1, C3 andC4—i.e., fixed- or direct-ratio.

Synchronous operation in FG2—the input speed and output speedrelationships where clutches C1, C2 and C4 are operating withsubstantially zero slip speed thereacross—is represented by line 93.Similarly, the relationships between input and output speed duringoperation in FG3, whereat clutches C2, C3 and C4 are operatingsimultaneously with substantially zero slip speed thereacross, isrepresented by line 95.

To the left of the shift ratio line 91 is an exemplary region ofoperation for the first EVT mode, EVT1, wherein both C1 and C3 areapplied, and C2 and C4 are released. To the right of the shift ratioline 91 and left of shift ratio line 93 is an exemplary region ofoperation for the second EVT mode, EVT2, wherein C1 and C4 are applied,and C2 and C3 are released.

To the right of shift line 93 and left of shift ratio line 95 is anexemplary region of operation for the third EVT mode, EVT3, wherein bothC2 and C4 are applied, and C1 and C3 are released. To the right of theshift ratio line 95 is an exemplary region of operation for the fourthEVT mode, EVT4, wherein C2 and C3 are applied, and C1 and C4 arereleased. As used herein with respect to clutches C1-C5, the terms“applied” or “actuated” indicate substantial torque transfer capacityacross the respective clutch. Antithetically, the terms “released” or“deactivated” indicate insubstantial or no torque transfer capacityacross the respective clutch.

While the regions of operation specified above may be generally favoredfor operation of the hybrid transmission 14, it is not meant to implythat the various EVT regions of operation depicted in FIG. 3 cannot ordo not overlap. Generally, however, it may be preferred to operate inthe specified regions because each particular mode of operationpreferably employs gear sets and motor hardware particularly well suitedin various aspects (e.g., mass, size, cost, inertial capabilities, etc.)for that region. Similarly, while the individual regions of operationspecified above are generally preferred for the particular modes ofoperation indicated, it is not meant to imply that the regions ofoperation for the individual EVT modes cannot be switched.

Generally, a shift into Mode 1 may be considered a downshift and isassociated with a higher gear ratio in accordance with the relationshipof N_(i)/N_(o). In contrast, a shift into Mode 4 is considered anupshift, and is associated with a lower gear ratio in accordance withthe relationship of N_(i)/N_(o). As discussed herein, other mode-to-modeshift sequences are feasible. For example, a shift from EVT1 to EVT3 isalso an upshift, while a shift from EVT4 to EVT2 is considered adownshift.

Referring now to the flow chart shown in FIG. 4, and with continuedreference to FIG. 1-3, a control algorithm for regulating operation of amulti-mode hybrid transmission is shown. The algorithm, process, ormethod 100 is capable of starting an internal combustion engine (12) ofa vehicle having a first electric machine (56) and a second electricmachine (58) incorporated into a multi-mode hybrid transmission (14).

For illustrative purposes, the method 100 is described with reference tostructures shown and described in relation to FIG. 1. However, thosehaving ordinary skill in the art will recognize other structures whichmay be used to practice the method 100 and the invention as defined inthe appended claims.

Method 100 begins at an initiation or start 102, which may coincide witha vehicle operator turning on the ignition of the vehicle, or maycoincide with another triggering event. During the entirety of method100, the hybrid control module and ECU 80 may be monitoring variousattributes of the vehicle and hybrid powertrain 10.

Step 104 occurs when the hybrid powertrain 10 is operating inelectric-only mode and the hybrid control module requests a high-voltagestart of engine 12. High-voltage engine starts are those in which ahigh-voltage electric machine (such as motor A or motor B) starts theengine 12, as opposed to a low-voltage starter motor powered by the12-volt SLI battery. Execution of the high-voltage engine start requiresthe transmission 14 be placed in one of the EVT modes.

Method 100 checks to see whether an engine reactive clutch (ERC), C3 orC4, is engaged in decision step 106. C3 and C4 are engine reactiveclutches because one of C4 and C3 must be engaged to activate the secondplanetary gearset 26 (P2) against power and torque from the engine 12.Otherwise, second planetary gearset 26 will freewheel and thetransmission 14 is unable to balance the input power—from engine 12,motor A, and motor B—against the output power delivered to final drive16. Note that motor A is always available to react the engine 12 withfirst planetary gearset 24 (P1), allowing torque transfer between theengine 12 and the first planetary gearset 24.

If either C4 has connected motor A to second planetary gearset 26(activating P2), or C3 has grounded the ring gear 40 of second planetarygearset 26 (activating P2) step 106 is answered affirmatively. Method100 proceeds to step 108 if an engine reactive clutch is engaged. Atstep 108 the high-voltage engine start is requested and either motor Aor motor B starts engine 12.

If neither of the two engine reactive clutches (C3 and C4) is engaged,method 100 proceeds to step 110, where the auxiliary pump is commandedon. Either C3 or C4 may be alternatively referred to as the first or thesecond engine reactive clutch. The auxiliary pump is used to actuate oneof the engine reactive clutches.

Method 100 determines which of the engine reactive clutches should beselected and actuated by determining, in step 112, whether aninput-split mode or a compound-split mode is desired. If an input-splitmode is requested, the method 100 proceeds to step 114 and synchronizesand commands the clutch C3, which is a grounded clutch (brake) and maybe referred to as the first engine reactive clutch. C3 is synchronizedusing either motor A or motor B and then engaged.

Decision step 116 verifies that the selected engine reactive clutch, C3,is successfully engaged. If C3 is properly engaged, the method 100 willproceed to step 108, where a high-voltage start of engine 12 occurs. IfC3 is not engaged, the method 100 will return to step 114 and againattempt to synchronize and command C3 so that the high-voltage enginestart can be enabled.

If a compound-split mode is requested, the method 100 proceeds from step112 to step 11 and synchronizes and commands the clutch C4, which is arotating clutch and may be referred to as the second engine reactiveclutch. C4 is synchronized using either motor A or motor B and thenengaged. Decision step 120 verifies that the selected engine reactiveclutch, C4, is successfully engaged. If C4 is engaged, the method 100will proceed to step 108, where a high-voltage start of engine 12occurs. If C4 is not engaged, the method 100 will return to step 118 andagain attempt to synchronize and command C4 so that the high-voltageengine start can be enabled.

At step 108, the hybrid controller may use either motor A or motor B tostart engine 12. Alternatively, the hybrid controller may use bothelectric machines (both motor A and motor B) to provide the requestedhigh-voltage engine start.

To provide the necessary power to start the engine 12 for thehigh-voltage start of step 108, motor A and motor B may be configured tooperate in conjunction with a high-voltage battery pack. Thehigh-voltage start therefore has greater potential energy availablecompared to a start with the 12-volt SLI starter. Furthermore, thetransmission 14 is configured such that motor A and motor B remainavailable as propulsion or traction motors for the vehicle during thehigh-voltage engine start.

While the best modes and other modes for carrying out the presentinvention have been described in detail, those familiar with the art towhich this invention pertains will recognize various alternative designsand embodiments for practicing the invention within the scope of theappended claims.

1. A method of starting an internal combustion engine of a vehiclehaving a first electric machine and a second electric machineincorporated into a multi-mode hybrid transmission, comprising:selecting one of a first clutch and a second clutch; pressurizing theselected clutch with an auxiliary pump; synchronizing the pressurizedclutch using at least one of the first and second electric machines;engaging the pressurized clutch; and starting the internal combustionengine with at least one of the first and second electric machines. 2.The method of claim 1, wherein the first and second clutches are enginereactive clutches.
 3. The method of claim 2, wherein selecting one ofthe first and second clutches includes: determining whether one of aninput-split mode and a compound-split mode is requested; and selectingthe first clutch if the input-split mode is requested, and selecting thesecond clutch if the compound-split mode is requested.
 4. The method ofclaim 3, wherein synchronizing the pressurized clutch includes usingboth of the first and second electric machines.
 5. The method of claim4, wherein both the first electric machine and the second electricmachine are used for starting the internal combustion engine.
 6. Themethod of claim 5, wherein the first clutch is a grounded clutch and thesecond clutch is a rotating clutch.
 7. The method of claim 6, whereinthe first and second electric machines are high voltage machines,configured to operate in conjunction with a high-voltage battery pack.8. The method of claim 6, wherein the first and second electric machinesare configured as propulsion motors for the vehicle.
 9. A method ofstarting an internal combustion engine of a vehicle having a firstelectric machine and a second electric machine incorporated into amulti-mode hybrid transmission, comprising: determining whether one ofan input-split mode and a compound split mode is requested; selecting afirst clutch if the input-split mode is requested, and selecting asecond clutch if the compound-split mode is selected; pressurizing theselected clutch with an auxiliary pump; synchronizing the pressurizedclutch with at least one of the first and second electric machines;engaging the pressurized clutch; and starting the internal combustionengine with at least one of the first and second electric machines. 10.The method of claim 9, wherein both of the first and second electricmachines are used to start the internal combustion engine.
 11. Themethod of claim 10, wherein the first and second clutches are enginereactive clutches.
 12. The method of claim 11, wherein the first clutchis a grounded clutch and the second clutch is a rotating clutch.
 13. Themethod of claim 12, wherein synchronizing the pressurized clutchincludes using both of the first and second electric machines.